1. Field of the Invention
The present invention relates to graphene electronic and/or photonic integrated circuits, and particularly to circuits having improved electrical contacts between a graphene film and metal electrodes.
2. Description of Related Art
Graphenes (also called graphene sheets) are a sheet of six-membered rings which does not form a closed surface, and are formed by connecting numerous benzene rings two-dimensionally. Carbon nanotubes are formed by rolling up a graphene sheet into a tubular structure. Graphites are formed by stacking multiple graphene sheets. Each carbon atom in a graphene sheet has an sp2 hybrid orbital, and delocalized electrons are present at opposite surfaces of a graphene sheet.
The following typical physical properties of graphenes have been reported: (a) The carrier mobility is in the order of 200,000 cm2/Vs, which is one order of magnitude higher than those of silicon (Si) crystals and is also higher than those of metals and carbon nanotubes. (b) The 1/f noises of typical nanodevices can be significantly reduced. (c) The refractive index is negative. (d) The surface electrons behave as if they have no mass. Because of these properties, graphenes are identified as a candidate for post-silicon electronic materials.
In order to realize graphene based electronic devices and optical integrated circuits, it is essential to establish good (e.g., low resistance) electrical contact between a graphene film and metal electrodes. Lee et al. reports the contact resistance between a carbon nanotube and a metal electrode (see, e.g., Jeong-O Lee, C Park, Ju-Jin Kim, Jinhee Kim, Jong Wan Park, and Kyung-Hwa Yoo: “Formation of low-resistance ohmic contacts between carbon nanotube and metal electrodes by a rapid thermal annealing method”, J. Phys. D: Appl. Phys. 33, 1953 (2000)). The contact resistance described in the above paper is in fact the parallel resistance of the electrical resistance of the carbon nanotube itself and the contact resistance between the carbon nanotube and the metal electrode. As used herein and in the appended claims, the term “contact resistance” includes such parallel resistance as described in the above paper. According to the above paper, the contact resistance between the carbon nanotube and the metal electrode is in the order of magnitude of kΩ (0.5 to 50 kΩ at room temperature).